176 The Rev. T. R. Robinson's Description of an improved Anemometer. 



direction at each hour. This is found by bisecting the arc of the hour-circle, 

 which is shaded by the pencil.* The mean direction during each hour will, 

 in general, not differ from the mean of those at its beginning and end ; but if the 

 eye perceives that this is not the case, those for the decimals of the hour may 

 be taken. From this are computed two rectangular co-ordinates, which are 

 given in the third and fourth columns ; w the motion of the wind from the west, 

 s that from the south. These are obtained by multiplying the hourly spaces into 

 the sine and cosine of the mean direction. I have found it easiest to do this by a 

 lartre sliding rule, having arranged a table of sines and cosines for each deci- 

 mal of the degree. They need only be to three places of decimals, but should 

 have a quadruple argument; its first column from 0° to 90°, its second from 180° 

 to 270' (these on the left): its third from 360' to 270°; its fourth from 180° to 

 90° (these on the right): and over each column the appropriate signs. Detaiils 

 of this kind may seem trifling, but the waste of labour which they avoid is of 

 great consequence when so great a mass of work has to be performed, as even 

 one year of such a registry involves. 



From these co-ordinates any final results may be obtained, as hourly, 

 daily, monthly, or yearly means. Let Whe such a mean of w, S of s, attending 

 to the signs ; then D, the mean direction for that time, and 2, the mean space, 

 are given by the equations 



„ TT ^, 5 W 



tan 1) = -pr-; i = 



6" cosD sinD' 



remembering that sinD has the same sign as W, and cosD as S, from which 

 the quadrant oiJD is known. 



As an example I annex the reductions of the twelve hours during which the 

 centre of the cyclone already referred to passed the Observatory, as one which 

 will illustrate the process in an extreme case. 



• This is most rapidly performed by a plan explained in the 

 figure. Let BC be the arc of the hour circle H ; lay an edge of 

 the ruler ET through C, and the centre I, so that its extreme 

 point is on the hour circle. Then lay the parallel -ruler PL through 

 that point and B; remove TE, and move the half of PL tUl it 

 passes through I ; the point G is in the line bisecting BC. „ 



